ACES (original) (raw)

ACES [ESA]

ACES (Atomic Clock Ensemble in Space) is an European Space Agency (ESA) ISS attached payload designed to operate a highly precise time and frequency reference system.

ACES carries two ultra-stable atomic clocks: PHARAO (Projet d'Horloge Atomique par Refroidissement d'Atomes en Orbite) and SHM (Space Hydrogen Maser). ACES enables precision time transfer experiments between space and ground stations, allowing tests of Einstein�s theory of General Relativity and searches for potential variations of fundamental constants. Data from ACES supports applications in geodesy, fundamental physics, and the synchronization of global timekeeping systems.

PHARAO is a primary frequency standard based on cold atoms, cooled with laser techniques to near absolute zero, offering very high stability and accuracy. SHM complements PHARAO by providing excellent short-term stability using a hydrogen maser. Together, these clocks create a highly reliable time reference in orbit. The ACES payload also includes a Microwave Link (MWL) and an optical communication terminal for precise time and frequency comparisons with ground stations.

ACES was launched aboard a SpaceX Dragon CRS-32 spacecraft on 21 April 2025. The payload will be mounted on the external platform of ESA�s Columbus module.